Strengthening sitalls

The role of defects in determining the strength properties of brittle materials is well known. However, the direct identification of defects in sitalls (glass-ceramics) that determine attained strength levels has not been achieved until now, due to the fact that such defects range from macroscopic to submicroscopic in nature. Structural defects (microcracks in the residual glass phase and on the phase boundary) can occur, as well as surface defects caused by machining, etc. The effectiveness of various strengthening methods was studied using industrial sitalls: AS-418 (Li203-SiO2-TiO2), AS-023 (Li2O-K2O-Al2O3-SiO2) AS-370 (MgO-Al2O3-SiO2-TiO2) and AS-380 (MgO-MnO-Al2O3-SiO2-TiO2). Conventional methods were used, namely etching and ionic exchange. A combined method of strengthening involving preliminary reduction of the surface defects by etching and subsequent ion-exchange treatment was found to be effective. The feasibility and effectiveness of chemical-thermal methods, in particular carbonitration and cementation, was also demonstrated for the first time. Results are given for the strengthening of AS-380 cordierite sitall using such methods. 7 refs.